1. Field of the Invention
This invention relates to control systems in which an instrument such as a key or card having a code recorded on it is submitted to a reader which reads the recorded code. Such control systems may be used in magnetic locks and various accounting and credit control facilities. In the case of a magnetic locking system the reader would control the portion of a lock and the coded instrument may be in the form of a key which is inserted into the reader. In the case of credit control systems the instrument would usually be in the form of a coded card.
2. Description of Prior Art
Australian patent application No. 65612/74 and granted U.S. Pat. No. 3,953,712 disclose a system in which a magnetically coded instrument is inserted into a guideway of a reader. The reader includes a number of sensing coils disposed adjacent to the guideway and electromotive forces are generated in those sensing coils by the movement of the magnetized portions of the instrument across them as the instrument approaches the end of its forward travel along the guideway. The electromotive forces generated in the sensing coils are amplified and influence the condition of an output circuit so as to cause generation of an output signal. This system has been generally satisfactory in service and has been commercially successful. However, it has been found that correct generation of potential difference signals in the sensing coils is dependent upon the speed at which the instrument is inserted into the guideway and some operator technique is required. The present invention provides a modification or improvement which avoids this difficulty. The modified system according to the present invention can also be more readily adapted to transmit signals in serial form rather than in parallel, which enables a simplification in the electrical wiring and generation of an output which is computer compatible.